G.B.
Unless the baby is a preemie, I think newborn size stuff is kind of a waste because they grow so ridiculously fast at that age. If you get newborn onesies, chances are they'll only use them for a couple of weeks before they outgrow them. Maybe just 1 pack of newborn size diapers and the rest in size 1. I tend to buy bigger sizes because cotton stuff shrinks and because I like buying everything big so they can grow into it. I don't think the mom's weight gain necessarily predicts the baby's weight. 25 lbs. is the low end of the range of recommended healthy weight gain but I knew someone who exercised throughout her pregnancy, only gained 12 lbs.(!) but delivered an average weight baby.
Also, clothing sizes are sooo inconsistent these days, even at the same store things that have the same size on the tag can be cut much bigger or smaller depending on the item. So you kind of have to eyeball it.